This application is related to Japanese Patent Application No. 2000-5280 filed on Jan. 5, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave filter. Filters utilizing a surface acoustic wave (referred to as SAW hereinafter) exhibit sharp filter characteristics and allow size reduction. Accordingly, they are now used for communication equipment such as portable phones and the like.
2. Description of the Related Art
Various designing techniques are known for SAW filters.
A construction of a transversal SAW filter is shown in FIG. 11. This transversal SAW filter is comprised of an exciting electrode 102 and a receiving electrode 103 which have a plurality of interdigital electrode fingers formed in a comb-like form with a period p on a piezoelectric substrate 101. Both the electrodes are typically formed of an aluminum alloy which usually has a low resistance and a small specific gravity. When an AC voltage 104 is applied to the exciting electrode 102, an electric field is generated and a distortion corresponding to the period p of the electrode fingers is generated on the piezoelectric substrate. This distortion is received as a SAW by the receiving electrode 103.
The central frequency f0 of the SAW excited by the exciting electrode 102 has the following relation with the period p and a velocity V of the SAW which is determined by the type of the piezoelectric substrate:
V=f0xc3x97p.
When the interdigital electrode is used as a filter, a band width, a degree of flatness in the band, a degree of suppression, a group delay time and the like are important characteristics. With the transversal SAW filter, these characteristics can be easily controlled by assigning a weight to its electrode. Accordingly, the transversal SAW filter is often used for an intermediate frequency (IF) filter, which permits some loss.
The transversal filter has a problem in that a ripple sometimes occurs in a passband resulting from a tertiary echo.
A generation mechanism of the tertiary echo is shown in FIG. 12. A main response received by a receiving electrode 103 is partially re-excited to the left in the figure by the receiving electrode 103 like reflection. This re-excited wave is further re-excited by an exciting electrode 102 and received by the receiving electrode 103 with a time delay. This phenomenon is the tertiary echo.
Here, re-excitation is referred to simply as xe2x80x9creflection.xe2x80x9d The tertiary echo is also referred to as triple transit echo (TTE). Because the reflection repeats again and again, the intensity of the tertiary echo is small as compared with the main response but it produces a ripple in the band in a frequency characteristic. Thus, the tertiary echo has been a cause of deterioration of the filter frequency characteristic.
The present invention provides a surface acoustic wave filter including a piezoelectric substrate; an exciting electrode formed on the piezoelectric substrate and exciting a surface acoustic wave; and a receiving electrode receiving the surface acoustic wave. Both the exciting and receiving electrodes are provided with interdigital electrode fingers having a period p. Either one of the exciting and receiving electrodes is constructed of a plurality of interdigital electrode sub-units having substantially the same electrode surface areas occupied by interdigital electrode fingers. The interdigital electrode sub-units are disposed in a direction perpendicular to a propagation direction of the excited surface acoustic wave and have a positional relation such that the interdigital electrode sub-units are offset from each other by (nxc2x1xc2xc)p, wherein n is an integer, in a direction parallel to the propagation direction of the surface acoustic wave.
A SAW filter of the present invention can suppress the ripple caused by the TTE.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.